Cerebrospinal fluid (CSF) is a clear, colorless and transparent fluid that circulates through ventricles of brain, subarachnoid space and central canal of spinal cord.

CSF is a part of ECF.

Major Topics

• Definition
• Properties and composition of CSF
• Formation of CSF
• Circulation of CSF
• Absorption of CSF
• Pressure exerted by CSF
• Functions of CSF
• Collection of CSF
• Blood-brain barrier
• Blood-cerebrospinal fluid barrier
• Applied physiology: Hydrocephalus

Properties

Composition of CSF

CSF contains more amount of sodium than potassium since it is a part of ECF.

CSF also contains some lymphocytes.

CSF secreted by ventricle does not contain any cell.

Lymphocytes are added when CSF flows in the spinal cord.

Water and Solids

Organic Substances

1. Proteins
2. Amino acids
3. Sugar
4. Cholesterol
5. Urea
6. Uric acid
7. Creatinine
8. Lactic acid

Inorganic Substances

1. Sodium
2. Calcium
3. Potassium
4. Magnesium
5. Chlorides
6. Phosphate
7. Bicarbonates
8. Sulfates

CSF is formed by choroid plexuses, which are tufts of capillaries situated within the ventricles of brain.

It is formed by the process of secretion in which involutes active transport.

Major quantity of CSF is formed in lateral ventricles and passes through foramen of Monro into the third ventricle.

From here, CSF passes to the fourth ventricle through aqueductus Sylvius.

From fourth ventricle, CSF enters into cisterna magna and cisterna lateralis through foramen of Magendie (central opening) and foramen of Luschka (lateral opening).

From cisterna magna and cisterna lateralis, CSF circulates through subarachnoid space over spinal cord and cerebral hemispheres.

It also flows into the central canal of spinal cord.

CSF is mainly absorbed by arachnoid villi into dural sinuses and spinal veins.

Small amount is absorbed into cervical lymphatics and perivascular spaces.

Pressure exerted by CSF varies in different positions.

1. Protective Function

CSF acts as a fluid buffer and protects the brain from shock.

Since, the specific gravity of brain and CSF is more or less same, brain floats in CSF.

When head receives a blow, CSF acts like a cushion and prevents movement of brain against the skull bone and thereby prevents damage of brain.

Contrecoup Injury

But severe blow affecting brain results in contrecoup injury.

Contrecoup injury is the injury to brain, in which damage is on side opposite to the side on which head receives a severe blow.

When the head receives a severe blow, brain moves forcefully and hits against skull bone, leading to damage of brain tissues.

Brain strikes against the skull bone at a point opposite to the point where blow was applied.

Hence the name contrecoup injury.

2. Regulation of Cranial Content Volume

Regulation of cranial content volume is essential because brain may be affected if the volume of cranial content increases.

Volume of cranial content increases in cerebral hemorrhage and brain tumors.

Increase in cranial content volume is prevented by greater absorption of CSF to give space for increasing cranial contents.

3. Medium of Exchange

CSF forms the medium through which many substances, particularly the nutritive substances and waste materials are exchanged between blood and brain tissues.

CSF is collected either by cisternal puncture or lumbar puncture.

Cisternal Puncture

In cisternal puncture, CSF is collected by passing a needle between the occipital bone and atlas, so that it enters the cisterna magna.

Lumbar Puncture

In lumbar puncture, the needle is introduced into subarachnoid space in lumbar region, between third and fourth lumbar spines.

Barrier exists in capillary membrane of all parts of the brain except in some areas of hypothalamus.

Structure of Blood-Brain Barrier

BBB is formed by tight junctions in endothelial cells of the brain capillaries.

Cytoplasmic foot processes of astrocytes (neuroglial cells) develop around capillaries and reinforce the barrier.

Functions of Blood-Brain Barrier

1. BBB acts as a mechanical barrier and prevents harmful chemical substances from entering brain.
2. BBB provides a healthy environment for brain tissues by preventing injurious amounts of organisms.
3. BBB permits metabolic and essential materials into brain tissues.

Substances Which Can Pass Through Blood-Brain Barrier

1. Oxygen
2. Carbon dioxide
3. Water
4. Glucose
5. Amino acids
6. Electrolytes
7. Drugs, such as L-dopa, 5-HT, sulfonamides, tetracycline and many lipid-soluble drugs
8. Lipid-soluble anesthetic gases, such as ether and nitrous oxide
9. Other lipid-soluble substances

Substances Which Cannot Pass Through Blood-Brain Barrier

1. Injurious chemical agents
2. Pathogens, such as bacteria
3. Drugs, such as penicillin and the catecholamines
4. Dopamine (so, parkinsonism is treated with L-dopa, instead of dopamine)
5. Bile pigments

This barrier exists at choroid plexus.

Function of this barrier is same as that of BBB.

It does not allow the movement of many substances from blood to cerebrospinal fluid.

It allows the movement of only those substances which are allowed by BBB.

Types and Causes of Hydrocephalus

1. Internal Hydrocephalus or Non-Communicating Hydrocephalus

Internal hydrocephalus is the accumulation of CSF in ventricles of brain caused by blockage of cerebral aqueduct.

It causes dilation of ventricles resulting in enlargement of head and cortical atrophy.

2. External Hydrocephalus or Communicating Hydrocephalus

External hydrocephalus is the accumulation of CSF in subarachnoid space.

There is dilatation of ventricles and widening of subarachnoid space resulting in enlargement of head.

It is due to blockage of arachnoid villi.

Features of Hydrocephalus

Hydrocephalus along with increased intracranial pressure causes headache and vomiting.

In severe conditions, it leads to atrophy of brain, mental weakness and convulsions.